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The crape myrtle (Lagerstroemia spp.) genus of trees and shrubs are reliably hardy in zones 6 to 9, depending on the species, variety or cultivar. Most often, mold problems, such as powdery mildew, associated with crape myrtle can be found primarily on its leaves, as can fungal spot. But it is possible for unsightly growth to take over the trunk of the tree itself.

Identifying Sooty Mold

Sooty mold looks much like it sounds -- as if black ashes had been rubbed over part of a plant. With crape myrtle, sooty mold does appear on leaves, but it can eventually cover the the trunk. Its host is honeydew, a substance excreted by aphids and some other insects. For this reason, sooty mold will often feel sticky. You can remove sooty mold and honeydew with a soap spray and soft brush, but to keep it from returning, you'll need to control the insects that leave the honeydew.

Aphid Control

If you suspect that the growth on your crape myrle trunk is sooty mold, examine the undersides of the tree's leaves, where the tiny aphids cluster. If there is an aphid infestation, blast the leaves, including their undersides with water to remove the aphids. Or kill the aphids with a homemade insecticide coat. Add 3 teaspoons of mild dish soap to 1 gallon plain water. Using a garden hose sprayer, coat the leaves thoroughly, especially lower leaves.

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Scale Control

A group of roundish, slow-moving scale insects looks so much like a lumpy growth that gardeners often mistake scales for a plant disease. As with aphids, treating crape myrtles for the pests is the best way to eradicate sooty mold. Existing scales can be washed off, along with sooty mold, using a soft-bristled brush and a pail of soapy water -- about 3 teaspoons mild liquid soap to 1 gallon water. Rinse the trunk after scrubbing it. A dormant oil spray can also be effective at controlling scales on woody ornamentals such as crape myrtles. When trees are just beginning to bud, mix 7 1/2 tablespoons of a dormant oil solution with 1 gallon of water. Coat the base of the tree and everywhere else scales congregate.

Considerations

Growths other than sooty mold can leave their mark on the trunks of crape myrtle trees. The most likely suspects are various fungal spores. These can develop in the center of the tree, but wounds or natural cavities on the outside of the trunk will encourage spores to reproduce there as well. Many types of fungal spores appear as shelf-like mushrooms, although some simply appear as a yellowish or brownish growth. Once the fungal growth has reach this stage, cutting down the tree may be the best option, especially if the crape myrtle is large and presents a danger to people or structures.

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About the Author

Ellen Douglas has written on food, gardening, education and the arts since 1992. Douglas has worked as a staff reporter for the Lakeville Journal newspaper group. Previously, she served as a communication specialist in the nonprofit field. She received her Bachelor of Arts from the University of Connecticut.